1. Field of Invention
The present invention relates to overload protection means for a series-excited parallel-loaded high-Q resonant L-C circuit.
1. Prior Art and General Background Considerations
Voltage-limiting non-linear resistance means, such as Varistors and Zener diodes, are commonly used to provide overload and/or overvoltage protection is situations where a high-Q resonant L-C circuit is series-excited from an AC voltage source and parallel-loaded with a load that, for one reason or another, may be or may become inoperative. However, with the load inoperative, the amount of power dissipated by the voltage-limiting means must by necessity be higher than the amount of power provided to the load when it is operative; and, when this power is relatively high--as may be the case in such applications as series-excited parallel-loaded power supplies for magnetrons and/or gas discharge lamps--the amount of power that has to be dissipated by the voltage-limiting means gets to be so large as to constitute a prohibitive proposition. Moreover, the energy waste associated with this dissipation may be considerable and should be avoided.
The following typical situation may help more specifically to illustrate the problem.
In powering a fluorescent lamp by way of a high-frequency series-resonant ballast, where the ballast constitutes a high-Q resonant L-C circuit series-excited from an AC voltage source and parallel-loaded by the fluorescent lamp, there is a serious problem associated with the situation where the fluorescent lamp for one reason or another ceases to constitute an effective load for this high-Q series-excited L-C circuit. In such a situation, which is most apt to occur toward the end of normal lamp life, the power drawn by the high-Q resonant L-C circuit from its AC voltage source is in most cases so excessively high as to cause damage to or even destruction of the L-C circuit and/or the AC voltage source. On the other hand, by connecting a voltage-limiting means, such as a Varistor, as an alternative parallel-load for the L-C circuit, damage to circuit and/or source may be prevented; however, the amount of power drawn from the source in case of an inoperative lamp will be so large as to represent an unacceptable level of energy waste--recognizing that it may often take a long time before a worn-out fluorescent lamp is replaced.